Sifakas are vertical clingers and leapers and use their tails as a
balancing organ in their movement strategy. Lack of a tail puts hominids
at a disadvantage at using this strategy, although we can to some extent.
(I remember once coming down a sage-brush-covered hill using this strategy
when I was eleven. I was lucky to keep on my feet all the way down. Of
course, that was also the summer I rode down the face of a flood control
dam on a bike... 8)